The present invention relates generally to refractory articles and compositions which are resistant to the erosive effects of molten steel as well as to hard refractory compositions which are resistant to abrasive wear. The present invention finds particular application in refractory pouring components, such as tubular nozzles and slide plates used in slide gate valves and in the continuous casting of steel.
In continuous steel casting operations, it is well-known to control the flow of molten metal from a ladle to a tundish and thence to the mold or molds by the use of a slide gate valve mechanism. The lowermost plate in such known slide gate valves carries the pouring nozzle. One of the refractory plates of a slide gate valve mechanism, sometimes referred to as the throttling plate, is movable so as to selectively bring the teeming bores in the refractory plates and pouring nozzle into and out of vertical alignment. When the teeming bores are in an axially aligned position, molten metal is free to flow and when the bores are in a completely misaligned position, all metal flow is stopped. In an intermediate or throttling position when the bores of the adjacent plates overlap such that their respective axes are offset, the molten metal flowing therethrough produces a turbulent flow zone downstream from the lowermost plate as a result of the restrictions caused by the offset bores. In present day steelmaking practice, it is also becoming more common to cast more chemically aggressive grades of steel which attack conventional materials and create an ever-increasing demand on the refractory components which contain and regulate the flow of the molten metal.
In the case of casting nozzles, the problem of refractory erosion is accelerated in the turbulent flow zone occurring in the upper portion of the nozzle bore when the slide gate plates are in the throttling position. This accelerated erosion caused by the turbulent flow and by the chemically aggressive steels results in premature failure in conventional alumina graphite refractory nozzles.
In addition, the flat surfaces of the refractory plates in slide gate valve mechanisms are subjected to high operating loads which cause abrasive wear when the plate surfaces slide relative to one another as the valve is opened and closed. It is, of course, advantageous to minimize such abrasive wear along the mating surfaces of adjacent refractory plates in order to prevent the infiltration of air into the molten metal stream which would have a deleterious effect on the quality of the steel being cast.
The prior art has recognized the need for providing casting nozzles with top plates or inserts along the top plate surface utilizing refractory materials of high hardness in an attempt to solve the abrasive wear problems alluded to above. It is also known to provide nozzle bores with bore sleeves or liners of various refractory compositions in an attempt to prevent or minimize the problem of nozzle bore blockage caused by the deposition and accumulation of alumina. Such refractory compositions used to prevent alumina deposition, however, are susceptible to erosion, particularly in the turbulent throttling zone in the upper region of the casting nozzle bore.
The present invention solves the problems heretofore encountered by providing a refractory composition and a casting member made therefrom which resists the erosive and corrosive effects of a turbulent molten steel. In addition, the refractory composition and casting member of the present invention possesses a hard, abrasive resistant surface which is particularly suited for use in the sliding plate surfaces of slide gate valves.